This particular error could be seen when saving a newsletter and the action is blocked without any possibility to continue.
The message is returned by a hosting provider filter which analyses the content of the sent data (actually your newsletter body, subject, and other parameters) and determines it contains unsafe things. Of course, that is not true but the filters cannot know you’re saving a newsletter.
The same problem may happen with other plugins as well and can be random: some newsletters are saved others not. Technically is a web-sense filter monitoring the data exchange between you and your site and trying to block suspicious behaviors.
How can it be solved?
This problem cannot be solved by the Newsletter plugin: you should ask the provider to remove the filter or configure it to not block the newsletters’ saving.
Sometimes it is a matter of adding directives in the .htaccess file, sometimes it is something that can only be changed or configured by the provider itself.
You can even check the provider console to see if you can remove that filter, or whitelist the /wp-admin URLs.
Why does it happen with newsletters?
A newsletter is a full HTML page, absolutely save to be stored into your database, but those filters believe sending a full HTML page from your browser (where you’re editing the content) to the site is bad. And stops it without any chance to continue.
Even some security plugins show this behavior. For example, sometimes you can see Wordfence blocking the saving of particular content, but their engine asks if you want to proceed and put in a “whitelist” that action.